Dead reckoning is the calculation of the current position of a movable object or person based on a previously determined position and a difference in that position using known, estimated or calculated speeds. Dead reckoning is, however, subject to cumulative errors. Gyroscopes have a drift error inherent to them that makes dead reckoning prone to large error accumulation. Gyroscope drift compensation is typically achieved by fusing IMU sensor data with compass heading or global positioning system (GPS) data. FIG. 1 refers to a prior art process for dead reckoning. In step 10, raw positional and orientation measurements are obtained from an accelerometer and gyroscope, based on changes compared to a known reference or given starting point. In step 12, the raw measurements are compensated for drift by taking into account GPS data and/or earth's magnetic field data. The process then outputs the dead reckoned data in step 14.
In an urban setting, however, the earth's magnetic field may be distorted, which may make a drift compensation calculation based on magnetic bearing inaccurate. Also, in urban settings, the GPS signal may be unavailable or be subject to reflections, which prevents a reliable fix from being obtained. Even away from urban settings, GPS signals may sometimes be unavailable or the earth's magnetic field may be subject to local distortions.
Energy may be harvested from the movement of body joints of humans and other animals by converting mechanical energy derived from such movement to electrical energy. Activities where body joints move repeatedly, such as walking, jogging, and running, for example, present opportunities to harvest energy from moving body joints over an extended period of time. FIG. 2 shows a person 20 wearing a prior art energy harvester 22 around his right knee joint. The power produced by a pair of energy harvesters may on average be 10 W, for example.
Energy may be harvested intermittently during a gait cycle, depending on the gait and which phase of a gait the user is in. FIG. 3 includes plots that are representative of various quantities relating to typical dynamics of a knee joint during a walking gait cycle 30. In graph A, plot 32 represents the angular velocity of the knee joint (i.e. the time derivative of the angle of the knee joint), where positive angular velocity represents movement in the knee extension direction and negative angular velocity represents movement in the knee flexion direction. In graph B, plot 33 represents the moment of the knee joint, where a positive moment represents torque in the extension direction and a negative moment represents torque in the flexion direction. Mechanical power associated with movement of the knee joint is the product of the torque (plot 33) and the angular velocity (plot 32) of the knee joint. Harvesting may be controlled by torque profiles that specify how much current to draw from the harvester at each point in the gait cycle. In some cases, an energy harvester may also supply power to the knee joint, to assist the user when tired.
Referring to FIG. 3, gait cycle 30 may generally be divided into a swing portion 36 and a stance portion 37. During the swing portion 36, the foot corresponding to the shaded knee (i.e. the right knee) is off of the ground. In the stance portion 37, the foot corresponding to the shaded knee is on the ground. Swing portion 36 may be further divided into a swing flexion portion 36A, during which the knee is flexing, and a swing extension portion 36B, during which the knee is extending. Stance portion 37 may be further divided into a stance/collision flexion portion 37A, during which the knee is flexing; a stance extension portion 37B, during which the knee is extending; and a swing flexion portion 37C, during which the leg is bending. During one gait cycle 30, angular velocity plot 32 comprises extrema 32A, 32B, 32C and 32D which occur, respectively, in swing flexion portion 36A, swing extension portion 36B, stance/collision flexion portion 37A and stance extension portion 37B. These extrema correspond to the end of acceleration of the knee joint.
Control logic in an energy harvester may include a finite state machine that has multiple states, each of which corresponds to a portion of repetitive motion of a body segment (e.g. a phase or portion of gait cycle 30). Simplified phases that the control logic may use when operating at least in part as a finite state machine are shown in FIG. 4. The states or phases of a complete gait cycle are shown, starting from swing extension 40, during which the subject leg is swinging from a bent position behind the body to a straight position in front of the body. At the end of the swing extension phase 40, i.e. when the heel strikes the ground 42, the collision flexion phase 44 starts. In the collision flexion phase the leg bends slightly at the knee, with the foot on the ground, during which the body's weight is transferred to the subject leg. This is followed by the stance extension phase 46, in which the subject leg straightens out while the foot is still on the ground, propelling the body forward. The following phase is the lowering flexion phase 48, in which the subject leg that is supporting the body's weight bends slightly in order for the other leg to reach forwards more before its heel strikes the ground 50. After the heel of the other leg has struck the ground 50, the swing flexion phase 52 commences, in which the subject leg is lifted from the ground behind the body, continuing the bending motion that started during the lowering flexion phase 48. After the subject leg has finished bending in state 52, the swing extension phase 40 starts again. The torque applied in each phase may either be positive or negative or both, depending on the assistance provided and/or whether energy is harvested. As can be appreciated, a more complex finite state machine, with more narrowly defined states or phases, may be used.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.